PEM fuel cells are well known in the art; as a power generation device, they convert chemical energy of fuels to electrical energy without their combustion and therefore without any environmental emissions. A PEM fuel cell like any electro-chemical cell of the stated categories, is formed of an anode and a cathode interposed by a layer of an electrolyte material for ionic conduction.
Embodiments of the conventional electro-chemical cell also include hardware components, e.g., plates, for reactant flow separation, current collection, compression and cooling (or heating). A separator plate provides multiple functions: (a) distributes reactant flow at the anode or cathode, (b) collects electrical current from operating anode/cathode surface and (c) prevents mixing or cross-over of the anode and cathode reactants in single cells. An assembly of two or more of these single cells is called a stack of the electro-chemical device. The number of single cells in a fuel cell stack is generally selected based on a desired voltage of the resulting stack. Conventionally desired voltages include 12 volts, 24 volts, 36 volts, 120 volts, and the like. For convenient assembly and/or dis-assembly of a fuel cell stack with large voltage or power output, multiple sub-stacks or modules, are combined to form the stack. The modules represent stacks of single cells in some number less than what ultimately results in the completed stack, as is well understood by those of skill in the art. When the stack forms a PEM fuel cell, such a module is often referred to as a PEM stack.
In the membrane electrode assembly (MEA) fuel (e.g., hydrogen) and oxidant (e.g., oxygen or air) react at the interfacial structures of the MEA to generate electrical power. Current HT MEAs are fabricated using a specific type of PEM material that contains phosphoric acid in its polymer matrix structure. The host matrix in the high temperature PEM material is thus a high temperature polymer material in which concentrated phosphoric acid is infused, and which is responsible for the proton conduction of the high temperature (e.g., 120° C. to 200° C.) PEM fuel cells. These HT PEM materials thus allow the fuel cell to operate at temperatures typical of phosphoric acid fuel cell (PAFC) operation (e.g., up to 200° C.) that are much higher than that of conventional low temperature PEM fuel cell operation (about 80° C.), which is typically well below 100° C.
While high temperature operation brings in a number of benefits, including but not limited to carbon monoxide (CO) tolerance, useful quality heat, fuel cell system simplification, and the like, the current (or power density per unit area) of the MEA is drastically reduced as compared to low temperature PEM MEAs, such as, e.g., MEAs made with Nafion® by E.I. du Pont de Nemours and Company. The power density per unit area of the MEA is reduced primarily due to intrinsic slow kinetics of oxygen reduction at the catalyst (typically Platinum)-phosphoric acid interface. More specifically, historical MEA technology development was targeted to the conventional low temperature PEM fuel cells in which the PEM material was Nafion® or equivalent perfluoro sulfonic acid ionomers.
Addition of these ionomer materials in the interfacial structure of MEAs has been utilized to enhance the power output of MEAs (e.g., S. srinivasan et al., J Power Sources, 22,359, 1988/29,367, 1990 and Mahlon S. Wilson et al., J. Electrochem. Soc., Vol. 139 No. 2, 1992). In U.S. Pat. No. 5,272,017, a proton conducting material was used to make a slurry of carbon supported catalyst particles; the slurry was applied to opposed surfaces of the PEM, which was then hot-pressed to embed at least a portion of the particles into the membrane. In a similar example in U.S. Pat. No. 5,882,810, the active layer of the MEA contained catalytically-active particles and an ionomer with lower equivalent weight than that of the PEM material itself. In yet another example, in U.S. Pat. No. 6,287,717 B1, an electrode comprising catalytically active metal particles and an ionically conductive polymer (ionomer) was bonded to an ionically conductive polymer membrane to form an electrode-membrane interface.
Current technology for high temperature MEAs containing phosphoric acid in the polymer matrix does not include addition of any ionomer in the catalyst layer. However, different acidic materials including perfluorinated sulfonic acid have been used as additives in the body of high temperature PEM membranes, particularly to enhance their proton conductivity.